Comparison between end-tidal CO2 and respiration volume per time for detecting BOLD signal fluctuations during paced hyperventilation

Respiratory motion and capnometry monitoring were performed during blood oxygen level-dependent (BOLD) functional magnetic resonance imaging (fMRI) of the brain while a series of paced hyperventilation tasks were performed that caused significant hypocapnia. Respiration volume per time (RVT) and end-tidal carbon dioxide (ETCO2) were determined and compared for their ability to explain BOLD contrast changes in the data. A 35% decrease in ETCO2 was observed along with corresponding changes in RVT. A best-fit ETCO2 response function, with an average initial peak delay time of 12 s, was empirically determined. ETCO2 data convolved with this response function was more strongly and prevalently correlated to BOLD signal changes than RVT data convolved with the corresponding respiration response function. The results suggest that ETCO2 better models BOLD signal fluctuations in fMRI experiments with significant transient hypocapnia. This is due to hysteresis in the ETCO2 response when moving from hypocapnia to normocapnia, compared to moving from normocapnia to hypocapnia.